[PATCH 2/2] maple_tree: Fix a potential memory leak, OOB access, or other unpredictable bug

[Peng Zhang]

在 2023/4/10 21:12, Liam R. Howlett 写道:
> * Peng Zhang <perlyzhang at gmail.com> [230410 08:58]:
>> 在 2023/4/10 20:43, Liam R. Howlett 写道:
>>> * Peng Zhang <zhangpeng.00 at bytedance.com> [230407 00:10]:
>>>> In mas_alloc_nodes(), there is such a piece of code:
>>>> while (requested) {
>>>> 	...
>>>> 	node->node_count = 0;
>>>> 	...
>>>> }
>>> You don't need to quote code in your commit message since it is
>>> available in the change log or in the file itself.
>> Ok, I will change it in the next version.
>>>> "node->node_count = 0" means to initialize the node_count field of the
>>>> new node, but the node may not be a new node. It may be a node that
>>>> existed before and node_count has a value, setting it to 0 will cause a
>>>> memory leak. At this time, mas->alloc->total will be greater than the
>>>> actual number of nodes in the linked list, which may cause many other
>>>> errors. For example, out-of-bounds access in mas_pop_node(), and
>>>> mas_pop_node() may return addresses that should not be used.
>>>> Fix it by initializing node_count only for new nodes.
>>>>
>>>> Fixes: 54a611b60590 ("Maple Tree: add new data structure")
>>>> Signed-off-by: Peng Zhang <zhangpeng.00 at bytedance.com>
>>>> Cc: <stable at vger.kernel.org>
>>>> ---
>>>>    lib/maple_tree.c | 16 ++++------------
>>>>    1 file changed, 4 insertions(+), 12 deletions(-)
>>>>
>>>> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
>>>> index 65fd861b30e1..9e25b3215803 100644
>>>> --- a/lib/maple_tree.c
>>>> +++ b/lib/maple_tree.c
>>>> @@ -1249,26 +1249,18 @@ static inline void mas_alloc_nodes(struct ma_state *mas, gfp_t gfp)
>>>>    	node = mas->alloc;
>>>>    	node->request_count = 0;
>>>>    	while (requested) {
>>>> -		max_req = MAPLE_ALLOC_SLOTS;
>>>> -		if (node->node_count) {
>>>> -			unsigned int offset = node->node_count;
>>>> -
>>>> -			slots = (void **)&node->slot[offset];
>>>> -			max_req -= offset;
>>>> -		} else {
>>>> -			slots = (void **)&node->slot;
>>>> -		}
>>>> -
>>>> +		max_req = MAPLE_ALLOC_SLOTS - node->node_count;
>>>> +		slots = (void **)&node->slot[node->node_count];
>>> Thanks, this is much cleaner.
>>>
>>>>    		max_req = min(requested, max_req);
>>>>    		count = mt_alloc_bulk(gfp, max_req, slots);
>>>>    		if (!count)
>>>>    			goto nomem_bulk;
>>>> +		if (node->node_count == 0)
>>>> +			node->slot[0]->node_count = 0;
>>>>    		node->node_count += count;
>>>>    		allocated += count;
>>>>    		node = node->slot[0];
>>>> -		node->node_count = 0;
>>>> -		node->request_count = 0;
>>> Why are we not clearing request_count anymore?
>> Because the node pointed to by the variable "node"
>> must not be the head node of the linked list at
>> this time, we only need to maintain the information
>> of the head node.
> Right, at this time it is not the head node, but could it become the
> head node with invalid data?  I think it can, because we don't
> explicitly set it in mas_pop_node()?
1. Actually in mas_pop_node(), when a node becomes the head node,
    we initialize its total field and request_count field.

2. The total field and request_count field of any non-head node,
    even if we initialize it, cannot be considered a valid value.
    Imagine if the request_count of the head node is changed, then
    we don't actually change the request_count of the non-head nodes,
    so it is an invalid value anyway.

>
> In any case, be sure to mention that you make a change like this in the
> change log, like "Drop setting the resquest_count as it is unnecessary
> because.." in a new paragraph, so that it is not missed.
I thought it was a small change that wasn't written in the changelog.
In the next version and any future patches, I will write down the
details of any changes.

Thanks.

>
>
>>>>    		requested -= count;
>>>>    	}
>>>>    	mas->alloc->total = allocated;
>>>> -- 
>>>> 2.20.1
>>>>